This invention relates to generally to engine air/fuel ratio control systems, and more particularly to air/fuel ratio control systems wherein such engine recovers fuel vapors which are purged from the fuel system and are fed to the engine.
Engine air/fuel control systems are known in which fuel delivered to the engine is adjusted in response to the output of an exhaust gas oxygen sensor to maintain average air/fuel ratios at a stoichiometric value. Such systems may also include a fuel vapor recovery system wherein fuel vapors are purged from the fuel system into the engine""s air/fuel intake. An example of such a system is disclosed in U.S. Pat. No. 5,048,493.
More particularly, current statutory regulations place a limit on the amount of fuel vapor that a passenger vehicle or light truck can emit while in operation or at rest. It is no longer acceptable merely to vent gasoline vapor to the atmosphere in order to relieve an accumulation of vapor due to high ambient temperature or heating of the fuel by proximity to sources of heat in the vehicle. The production of vapor can, in principle, be minimized by careful design, but cannot be entirely eliminated when certain conditions are present. Pressure build-up is reduced by storage of vapor on the surface of a material with high surface area, typically activated charcoal. In addition, to control the amount of vapor accumulated, modern automobiles carry out an operation called purge, in which vapor from the fuel tank and the storage canister is ingested into the engine, where it adds to fuel delivered by the usual fuel injection process. The purge process has the effect of adding both fuel and air to that supplied as part of the usual engine control strategy. In general, the delivered fuel and air are unmetered, because precise metering would entail both a flow meter and a sensor capable of measuring the fuel concentration in the purge flow. The addition of unmetered fuel and air complicates the control of the ratio of air to fuel (usually termed A/F), a quantity which must be closely regulated for minimum emissions. In extreme cases, excessive fuel from purge may cause the engine to stall. In the absence of a compensatory mechanism, the entire burden of handling the effects of purge is left to the usual closed-loop air/fuel (A/F) control strategy. In general terms, an excess of fuel due to purge will be handled by a reduction of injected liquid fuel; conversely, an excess of air (e.g., caused by a purge with minimal concentration of fuel) will be handled by an increase in the injected fuel. In neither case is the tendency of the added air flow to increase engine speed countered, nor is the effect of this flow on other parts of the strategy that depend on the knowledge of the mass flow of air taken into account. A further complication arises from the fact that some control strategies make diagnostic use of the value or pattern of values of the prime control variable in the A/F strategy. When the effect of purge is handled by the closed-loop control strategy, this control variable may spend considerable time at otherwise unusual values, thereby complicating diagnostic inferences.
To mitigate any deleterious effects of purge as just described, it is common to employ a purge compensation strategy. Compensation for purge fuel is frequently implemented by subtracting a term from the calculation of fuel for each cylinder event. This term is intended to be an estimate of the fuel per event provided by the purge flow. The underlying principle is the following: if this term is correct, then the average value of the internal estimate of the A/F control variable will be equal to its nominal value. In typical control strategies the A/F control variable is called LAMBSE, and its nominal value is unity. More particularly, LAMBSE is at an average value of unity when engine is operating at stoichiometry and there are no steady-state air/fuel errors or offsets. For a typical example of operation, LAMBSE ranges from 0.75-1.25. In typical closed-loop control, the value of LAMBSE is driven in an oscillatory fashion on the basis of an exhaust gas oxygen (EGO) sensor. Hence, the value of the purge term in the fuel calculation is adjusted in a direction such that the mean value of LAMBSE tends toward unity. This is usually performed essentially as a simple integral controller, in which the difference between LAMBSE and unity is integrated (accumulated), multiplied by a chosen constant parameter, converted to units of fuel injected per event, and inserted (subtractively) into the fuel calculation. In this method of computing fuel compensation, the control variable LAMBSE is effectively treated as the output of a system for which the compensation value is the control input.
The inventors herein have discovered numerous problems with prior air/fuel-purge compensation control systems. More particularly, the inventors have recognized that with the above described method of computing fuel compensation, the control variable LAMBSE is effectively treated as the output of a system for which the compensation value is the control input. Considered from this viewpoint, the system contains a delay between the time of application of the control input and the time of consequence of this input as observed at the system output. As is usual when such a system is treated with simple integral control, the integral control coefficient must be chosen to be small enough to avoid instability. Such instability could manifest itself, for example, as oscillations of system input and output. The practical consequence is that the extra fuel that is present upon initiation of the purge operation is compensated only after a significant time has elapsed. The disruptive effect of this lag in compensation may be partially mitigated by opening the purge valve slowly rather than rapidly. Unfortunately, if this is done, the time required to purge the fuel stored in the canister increases. In some cases this may pose a difficulty, since other required aspects of engine control and diagnostics are best performed when purge is not in operation. Another disadvantage of the simple integral compensation method is that a correct estimate of the fuel content of the purge stream occurs, at best, only in steady state when the error in the mean value of LAMBSE has been reduced to zero. This is a direct consequence of not treating delays explicitly.
In accordance with the present invention, a method is provided for controlling an air/fuel ratio of an engine, such engine being supplied fuel from a fuel injection system to inject fuel into a cylinder of such engine. The method includes providing a model of the engine. The model represents a relationship between: (1) a signal model LAMBSE, representative of estimated air/fuel ratio of the engine relative to a stoichiometric air/fuel ratio for the engine; and, (2) fuel injected into the cylinder of the engine. Exhaust gas oxygen emission from the engine is measured during operation of such engine. Actual LAMBSE produced by such engine during operation of such engine is produced as a function of such measured oxygen. The actual LAMBSE is compared with the model LAMBSE provided by the model in response to fuel injected into the engine to produce a model error signal. The fuel injected into the engine is adjusted in accordance with the error signal.
In one embodiment, the adjusting includes providing a reference LAMBSE signal. A model inverse to the first-mentioned model is also provided. The error signal is compared with the reference LAMBSE signal to produce a second error signal. The second error signal is fed to the inverse model to generate the fuel signal for the engine. The fuel signal is fed to the first-mentioned model to provide the model LAMBSE signal.
In one embodiment, the first-mentioned model includes a first section representative of a delay-free model of the engine and a second section. The second section represents a delay in the engine between a time a change in the fuel is injected into the engine and a time a change in the oxygen in the exhaust emission from such change in fuel is sensed. The method combines the delay free model output signal with the second error signal to produce the signal fed to the inverse model.
In one embodiment, the method combines an output of the second section with the actual LAMBSE to produce the first-mentioned error signal.
In one embodiment the first-mentioned model is a linear model.
In one embodiment the delay free section is represented as: ym=1+Gmu, where Gm is the gain of the model, u is the input to the model and ym is the output of the model.
In one embodiment, reference LAMBSE signal includes a feedforward air/fuel ratio control signal in accordance with anticipated fuel through the purging system.
In one embodiment the producing the feedforward air/fuel ratio control signal comprises determining fuel flow rate through the purge system.
In one embodiment the purge system includes a valve, such valve passing the fuel in the purging system to the intake manifold at a rate related to a duty cycle of a control signal fed to such valve. The flow rate through the purge system is determined in response to the duty cycle the control signal fed to the valve.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.